In music, a key is a set of notes that are arranged in a specific order and used as the basis for a musical composition. The key of a piece of music determines the pitch of the notes and the overall tonality or mood of the piece.

Each key is determined by a specific root note, also called the tonic, which is the starting point for the scale of notes that make up the key. For example, if a piece of music is in the key of C major, the root note is C and the notes in the scale are C, D, E, F, G, A, and B. The notes in a key are arranged in a specific order and have a specific relationship to each other, which creates a unique sound or tonality.

There are major keys and minor keys, which have different tonalities and moods. Major keys have a brighter and more joyful sound, while minor keys have a darker and more melancholy sound. The key of a piece of music can also affect the difficulty level for musicians, as different keys require different fingerings and techniques on instruments.

Musical keys are an important part of music theory and composition, and understanding them can help musicians create and analyze music more effectively.